It is necessary to first describe the operation of a conventional semiconductor to fully understand the present invention.
FIG. 5 is a circuit diagram of the major part of a conventional semiconductor device that uses a semiconductor element with a current sensing function. The semiconductor device is comprised of two components, a semiconductor element 6 and a control circuit 7. The control circuit 7 monitors and protects the semiconductor element 6 against over-current and short-circuit surges.
In FIG. 5, the semiconductor element with a current sensing function is shown as an IGBT 6 with a sensing function. Control circuit 7 is connected to IGBT 6.
The IGBT 6 is disposed with a collector terminal 6a, acting as a main electrode, an emitter terminal 6b, acting as another main electrode, a gate terminal 6c, acting as a control electrode, and an output terminal 6e for the current-sensing electrode 6d. This IGBT 6 turns a load device (not shown) on and off according to a drive signal 7a outputted from control circuit 7. When the IGBT 6 receives the drive signal 7a as an "on" signal, a main current 6f flows from the terminal 6a to the terminal 6b and powers the load device. When the IGBT 6 receives the drive signal 7a as an "off" signal, the main current 6f is turned off and stops powering the load device.
When the main current 6f flows from the IGBT 6, a sensing current 6g with a value proportional to the main current 6f flows from the current-sensing layer 6d to the output terminal 6e of IGBT 6.
FIG. 6 shows the composition of the aforementioned IGBT 6. As shown in FIG. 6, the IGBT 6 has an emitter layer 63 with a large area formed on the surface of a semiconductor substrate 61. This surface also contains a gate layer 64 and current-sensing layer 6d, whose surface area is smaller than that of the emitter layer 63. The collector layer (not shown in FIG. 6) is formed on the layer opposite the emitter layer 63.
The control circuit 7 consists of a drive circuit 71, a current-sensing resistor 72, a gate resistor 73 disposed as required, an over-current control circuit 74, and a short-circuit control circuit 75.
The operation of control circuit 7 is as follows:
The drive circuit 71 uses as its input a signal 7b generated by a control circuit at a previous stage, which is not shown, and a deactivating signal 7f generated by over-current control circuit 74. In the normal operation, the drive circuit 71 generates the drive signal 7a to turn the IGBT 6 on and off according to the activating signal 7b via the gate resistor 73.
In addition, when under certain circumstances, the deactivating signal 7f is generated and inputted to the drive circuit 71, the latter causes the signal 7a an "off" signal for the IGBT 6, regardless of what value to become signal 7b is, and forces the voltage between the emitter 6b and the gate 6c nearly zero, turning IGBT 6 off.
Current-sensing resistor 72 has a resistance value R.sub.s, and generates a detection voltage V6, proportional to the flowing sensing current 6g which in turn is proportional to the main current 6f.
The over-current control circuit 74 includes a comparator 741 and a reference voltage source 742, which provides a reference voltage E1 used to make determinations in the comparator 741. The reference voltage E1 is selected at a voltage value that is higher than the detection voltage V6 corresponding to the value of the main current 6f when the IGBT 6 is in the normal operation.
If the value of the detection voltage V6 exceeds the value of the selected reference voltage E1, the over-current control circuit 74 is triggered and generates the signal 7f to deactivate the drive circuit 71, thereby stopping main current 6f from flowing to the load device.
The short-circuit control circuit 75 consists of an NPN transistor 751 and a Zener diode 752 which is used as a constant voltage diode. The anode of diode 752 is connected to the collector of the transistor 751; its cathode is connected to the drive signal 7a, and the base of the transistor is connected to the current sensing resistor 72. With this arrangement, the detection voltage V6 across the resistor 72 can be sensed.
If the value of the detection voltage V6 exceeds the threshold voltage (V.sub.th) of the transistor 751, this transistor will be turned on, while the voltage between the gate terminal 6c and the emitter terminal 6b of the IGBT 6 is reduced to the constant break-down voltage value of the Zener diode 752.
With the above arrangements, the control circuit monitors and protects against any increases in the main current 6f above the normal level. If, for any reason, during the normal operation the value of the main current 6f increases, causing the value of the detection voltage V6 across current-sensing resistor 72 to exceed the reference voltage E1, the over-current control circuit 74 will detect this condition and generate the deactivating signal 7f to the drive circuit 71. The drive circuit 71, upon receiving the deactivating signal 7f, would switch the drive signal 7a to a low (L) level signal, forcing the IGBT 6 to turn off and stop the over-current.
In addition, if the value of main current 6f increases abruptly as a result of a short-circuit of said load device, the signal transmitted from over-current control circuit 74 cannot reduce the main current via drive circuit 71 instantaneously because of the time delay in the signal transmission along the transmission path. This would allow an excessive current 6f to continue to flow from IGBT 6 into load device.
In such a case, the short-circuit control circuit 75, which has a higher response speed, would detect the sharp increase in the main current 6f, and send its output signal directly to IGBT 6 without going through drive circuit 71. When the detection voltage reaches the predetermined level, the transistor is turned on and the Zener diode 752 breaks down to produce a constant voltage level. This quickly reduces the main current 6f to a value in response to the constant voltage diode that IGBT 6 can tolerate.
The main current remains subdued until IGBT 6 is turned off according to drive circuit 71 caused by a deactivation signal 7f from the over-current control circuit 74.
Through the above measure, the IGBT 6 and its load device are protected from main current 6f which exceeds normal current levels, making it possible to properly operate the system.
The main current value is altered when either the over-current control circuit 74 or short-circuit control circuit 75 are triggered by an over-current value. The value of the main current 6f obtained when the over-current control circuit 74 is triggered and generates an over-current detection signal 7f, is denoted I.sub.oc. The value of the sensing current 6g at the time of over-current is denoted i.sub.oc. The ratio between the two current values is denoted N. The resistance value in the current-sensing resistor 72 is R.sub.s and the reference voltage used to compare signals is denoted E1. Using these defined variables, the following formula 1 can be formulated: EQU E1=i.sub.oc .times.R.sub.s EQU i.sub.oc =I.sub.oc /N EQU I.sub.oc =N.times.E1/R.sub.s 1
The value of the main current 6f obtained when the NPN transistor 751 of the short-circuit control circuit 75 is turned on by a value defined at the short-circuit current detection level is I.sub.sct. The value of the sensing current 6g at this time is denoted i.sub.sct. The threshold voltage for the transistor 751 is V.sub.th. The following formula 2 can then be formulated: EQU V.sub.th =i.sub.sct .times.R.sub.s EQU i.sub.sct =I.sub.sct /N EQU I.sub.sct =N.times.V.sub.th /R.sub.s 2
Because the conventional control circuit 7 commonly uses the detection voltage V6 generated by sensing current 6g at the resistor 72 as the input signals of both the over-current control circuit 74 and the short-circuit control circuit 75, the former acting according to formula 1 and the latter acting according to formula 2, the ratio I.sub.oc /I.sub.sct of these two current values is equivalent to the ratio E1/V.sub.th of the two voltage values.
Therefore, once the reference voltage E1 and threshold voltage V.sub.th are determined, neither current value, I.sub.oc or I.sub.sct, can be set independently because the ratio must be equal to E1/V.sub.th.
For this reason, if, for example, the value, I.sub.oc, is increased so that a device may be used with the main current 6f up to a larger permissible value, the current value, I.sub.sct, must also be increased. If the main current 6f increases suddenly because of a short-circuit or some other failure, the IGBT 6 and load device will be subject to the risk of a current break-down.